The invention relates to control systems for automatic power transmissions of the kind providing forward gear ratios, neutral and reverse gear ratios in which forward ratio changes are effected under power by releasing one clutch inter-connecting the transmission input and output through one engaged gearset simultaneously with engaging another clutch inter-connecting the input and output through another engaged gearset.
In automatic transmissions of this kind, it is normally proposed to provide electronic sensing of factors such as road speed and engine torque and from these to derive electrical signals for gear ratio selection. It is also normal to provide one hydraulically operated gear actuator for each ratio which in turn normally requires one actuator for each gearset. Power for the actuators is derived from an engine-driven pump and the control of fluid under pressure from the pump to gear actuators is effected by electro-hydraulic valves which are normally referred to as solenoid valves. Unless one solenoid valve is provided for every gear actuator, it is normal to provide one or more spool valves between the solenoid valves and the gear actuators so that some actuators are operated in response to a combination of opened solenoid valves. In this way, with a conventional arrangement, four solenoid valves are required for a transmission with four forward ratios, neutral, one reverse ratio and provision for temporary engagement of two forward ratios while changing gear.
The solenoid valves employed are relatively expensive and expense is a particularly important factor in automatic transmissions intended for motor vehicles.
An object of the present invention is to provide a control system for an automatic transmission in which the number of electro-hydraulic valves can be reduced as compared with the known arrangement described above.